1. Field of the Invention
The present invention relates to an improved process for preparing styrenic polymer particles, particularly foamable styrenic polymer particles, having a substantially uniform particle size.
2. Description of the Prior Art
Styrene polymers in particulate form are conventionally prepared by a suspension polymerization process. However, such a process does not produce particles having a narrow size distribution. Particles having a narrow size distribution are desirable in view of their many advantages such as facilitating the supply of particles to a molding machine in unvarying quantities, producing moldings of uniform quality, and producing foamable particles which expand uniformly. Therefore, it is desirable to produce styrenic polymer particles having a narrow size distribution in an efficient manner.
British Pat. No. 1,255,237 discloses a suspension polymerization process for obtaining styrenic polymer particles of uniform particle size by dispersing previously prepared polymer particles having a small and uniform particle size (e.g. particles which have been sieved and collected) into an aqueous suspension, adding monomer containing a polymerization initiator to said suspension while stirring, and polymerizing the monomer within the polymer particles which act as nuclei for absorbing the monomer.
The above "seeding" process, however, does not produce as narrow a particle size distribution as is desired, and is particularly disadvantageous in that it tends to produce a large portion of fine powdery polymer. Such "fines" have no useful value, are difficult to recover and dispose of, and increase the cost of production.
Since this basic "seeding" process was first disclosed, there have been several attempts to eliminate some of the disadvantages associated with it. In U.S. Pat. No. 4,085,169 the patentee sought to reduce the quantity of fines produced in the seeding process by adding dropwise to the aqueous suspension of polymer particles a solution of styrene monomer, polymerization initiator and a polymerization retarder having a molecular weight of at least 200. It was theorized that the polymerization retarder inhibited the polymerization of the styrene monomer while in the fine oil droplet state, but allowed polymerization to proceed once the monomer was absorbed in the seed particles.
In U.S. Pat. No. 4,091,054 the patentees also sought to reduce the quantity of fines produced in the seeding process. Their attempt involved the simultaneous but separate dropwise addition of an initiator solution and a monomer solution to the suspension of seed particles. The initiator solution comprised 60-100% of the initiator in 1-10% of the monomer and the monomer solution comprised 0-40% of the initiator in 90-99% of the monomer.
It has been found that the above attempts at reducing the quantity of fines have introduced other problems to the seeding process. For example, the presence of polymerization retarder as called for by the first method is deleterious to the fusability of expanded particles during molding processes involving the application of heat and pressure. And the second method's task of preparing separate monomer and catalyst solutions and controlling their simultaneous but separate addition can be cumbersome and difficult. Also, the handling and storage of concentrated initiator solutions is potentially hazardous.